The invention is based on a sensor element for limiting current sensors for determining the .lambda. value of gas mixtures, particularly of exhaust gases of internal combustion engines. In such sensor elements, which employ the principle of the diffusion limiting current, the limiting current is measured at a constant voltage applied to both electrodes of the sensor element. In exhaust gas produced in combustion processes, this current is dependent on the oxygen concentration as long as the diffusion of the gas to the pump electrode determines the rate of the reaction occurring. It is known to construct such sensors employing the polarographic principle of measurement in a manner such that both the annode and the cathode are exposed to the gas to be measured, the cathode having a diffusion barrier in order to achieve operation in the region of the diffusion limiting current.
The known limiting current sensors serve, as a rule, to determine the .lambda. value of gas mixtures, which denotes the "total oxygen/oxygen required for complete combustion of the fuel" ratio of an air/fuel mixture combusting in a cylinder. The sensors determine the oxygen content of the exhaust gas by a change in electrochemical potential.
Owing to a simplified and cheap method of manufacture, the manufacture of sensor elements produced by ceramic-film and screen-printing technology has become established in practice in recent years.
In a simple and efficient manner, planar sensor elements can be produced on the basis of planar solid electrolytes or oxygen-conducting solid electrolytes in the form of a film, for example, from stabilized zirconium dioxide, which are coated on both sides with one inner and outer pump electrode each, having the associated conductor tracks. At the same time, the inner pump electrode is advantageously disposed in the peripheral region of a diffusion channel through which the gas under test is supplied and which serves as a gas diffusion resistor.
From German Offenlegungsschrift 3,543,759 and also EP-A-O, 142,993, O, 188,900 and O, 194,082, sensor elements and detectors are furthermore known which have in common the fact that they each have a pump cell and a sensor cell which comprise planar solid electrolytes or oxygen-conducting solid electrolytes in the form of a film and two electrodes disposed thereon and have a common diffusion channel.
A disadvantage of the prior art sensor elements is that the front section, facing the gas under test, of the inner pump electrode is subjected to a severer load than the rear section facing away from the gas under test. This results in a high electrode polarisation which requires a high pump voltage. The latter in turn entails the danger of an electrolyte decomposition in the region of the inner pump electrode.